Within the recent past a solid state Tacan antenna has been developed and accepted for shipboard installation. This antenna is an electronically scanned all band (252 channels) antenna system.
In earlier conventional Tacan antennas prior to this development, a rotating inner cylinder carrying a single parasitic element distorts the central antenna elements circular radiation pattern into a cardioid shape at a 15 revolution per second rate, producing the 15 cps modulation of the course bearing signal. There is a direct correspondence between electrical phase and geographical bearing; the single lobed cardioid pattern changes phase in a clockwise direction one degree with each one degree of geographical bearing.
This earlier Tacan antenna also has an outer rotating element, which rotates integrally with the inner cylinder at the same 15 revolutions per second rate. The outer cylinder, however, has nine elements embedded in it, in the form of vertical wires spaced uniformly at 40.degree. intervals. These wires--also parasitic elements--further distort the 15 cps cardioid pattern, producing a ninth harmonic, 135 cps sine wave component that provides the fine bearing indications.
This 135 cps component moves through a complete cycle, or 360 electrical degrees, for each 40.degree. arc of geometrical bearing so that the fine bearing feature produces a 9.degree. change in measured phase for each single degree of bearing change. Thus, a "magnifying" effect produces fine bearing indications.
Of course, the fine bearing indication has a 9-fold ambiguity--the phase would be identical for any of 9 directions that lie 40.degree. apart. To resolve the bearing ambiguity, the course 15 cps signal first moves the air-borne indicator into the correct 40.degree. bearing sector after which the fine 135 cps signal precisely positions the indicator within the sector. Once each revolution of the cylinders, a special coded course reference pulse is sent out to serve as a timing reference for the measurement of the phase of the pulse envelope wave. For further details of this earlier conventional Tacan antenna reference is made to the article entitled, "New Tacan Antennas Offer Gains" by Kenneth J. Stein published in the July 24, 1972 issue of Aviation Week & Space Technology.
These mechanically rotated Tacan antennas, although simple and reliable, are bulky, heavy and consume relatively large amounts of primary power.
The basis of the non-rotating electromagnetic wave energy transmitting antenna is the Yagi array disclosed in U.S. Pat. No. 1,860,123 granted May 24, 1932. In a Yagi-type array, several parallel planar dipoles are present including, in order, a not-fed dipole called reflector, a fed dipole called driven dipole and a number of non-fed suitably spaced parasitic dipoles called directors. The Tacan antennas of the non-rotating type are further disclosed in U.S. Pat. Nos. 3,560,978 granted Feb. 2, 1971; 3,845,485 granted Oct. 29, 1974; 3,846,799 granted Nov. 5, 1974; 3,863,255 granted Feb. 2, 1971 and 4,014,024 granted Mar. 22, 1977.
In the electronically scanned Tacan antennas a lightweight, compact configuration is advantageously afforded. The technique employed to achieve the characteristic 15 cps and 135 cps modulation components employs digital control of parasitic elements. A select number of parasitic elements are arranged around the central radiator, and these parasites are digitally switched in a predetermined pattern. The parasitic elements are small dipoles which are effectively detuned by large inductances to prevent current flow. The outer array of parasitics produce the 9th harmonic, 135 cps fine bearing modulation. This electronically scanned Tacan antenna is also described in the foregoing article and is available commercially from the Avionics Division of ITT, Nutley, New Jersey 07110 .
The solid state Tacan antenna offered by ITT in its shipboard configuration consists of two major units: an antenna assembly and a control monitor. These units together with the shipboard beacon, provide aircraft with distance and bearing information needed to determine their positions with respect to the ship.
The antenna assembly is designed for installation at the top of a mast. The antenna consists of three major subassemblies: (1) Rf subassembly, pedestal and an electronic subassembly. The RF subassembly is protected by a fiberglass honeycomb randome attached to a lower aluminum sect-on by quick release fasteners.
The RF assembly has replaceable parasitic modules arranged in a circular pattern on an aluminum honeycomb sandwich counterpoise. The inner ring consists of replaceable 15 Hz modules arranged in a circle around a central radiator.
With the acceptance of the ITT solid state electronically scanned antenna system, it was considered advantageous to utilize this configuration and provide for pattern synthesis and shaping for the countless applications envisioned for shipboard use as well as shore illustrations.
Furthermore, it would be extremely desirable to integrate L-band systems along with Tacan. Most of the L-band spectrum has been allocated to aeronautical radial navigation, communications and identification. Within these frequency allocations, many systems have been developed and are in various development status ranging from operational to advanced development. Therefore, electromagnetic waves propagation in the L-band spectrum coupled with Tacan would have many applications.